We have shown that critically-timed stretch of the intact left ventricle can reproducibly initiate ventricular arrhythmias. The cellular mechanism(s) are incompletely elucidated, but our preliminary data suggest a role for cardiac stretch-activated channels in the genesis of these arrhythmias. Since intracellular Ca++ ([Ca++]i) may be altered by muscle length and loading and since the dilated ventricle is particularly sensitive to stretch-induced arrhythmias, we hypothesize that [Ca++]i modulates stretch-induced arrhythmias; this effect of [Ca++]i may be mediated by calcium entry through stretch-activated channels or by length- dependent changes in [Ca++]i transduced by stretch or other mechanisms. In this Project, we will test these hypotheses in our isolated canine heart model in which the timing, duration, and amplitude of left ventricular volume changes can be precisely varied to elicit stretch-induced arrhythmias, and in in vitro studies in which the effects of timed, graded mechanical stretch on parameters such as conduction velocity and longitudinal resistance will be determined in axisymmetric isolated cardiac tissue preparations. Intracellular Ca++ transients will be measured directly using the Ca++-sensitive photoprotein, aequorin. The blockers nifedipine and ryanodine will be used to differentiate between the roles of gated Ca++ entry and release of Ca++ from sarcoplasmic reticulum. The effects of gadolinium, a putative blocker of stretch-activated channels which produces dose-dependent inhibition of stretch-induced arrhythmias in our model, will be studied. the hypothesis that Ca++ overload sensitizes the ventricle to the arrhythmogenic effects of stretch will be tested in acute and chronic models of intracellular Ca++ overload. the mechanism by which specific antiarrhythmic strategies influence ventricular contractility will be examined using load-independent measures of contractile state and direct measurements of [Ca++]i. The potential negative inotropic effect of Na+ channel block (lidocaine) and positive inotropic effect of K+ channel block (the Class III antiarrhythmic action) will be determined. Our studies are likely to provide new knowledge regarding basic mechanisms of arrhythmogenesis which are particularly relevant to patients with dilated and/or Ca++ overloaded ventricles, who are at high risk of serious ventricular arrhythmias and sudden death.